Due to the decreasing petroleum resource, it has become an inarguable fact that the crude oil becomes heavier and inferior. As a main processing means of converting the heavy oil to light fuels such gasoline and diesel oil, it is inevitable for the catalytic cracking to treat more inferior raw heavy oil more and more. The decreased conversion and the increased coke yield are caused by the influence of the inferior raw oil on the catalytic cracking. Therefore, many researches are conducted in the aspects of modifying the zeolite, producing the catalyst, and designing the process (Liu Tao, Zhang Zhongdong, Zhang Haitao, Sino-Global Energy. 2009, 14(1):71-77). However, for most of the current catalyst designing and industrial operation, the high yield of light oils and LPG is achieved by increasing the conversion for the cracking reaction, and therefore the coke yield is remarkly increased. This results in wasting the petroleum resource. In order to improve the utilization of heavy oil, the conversion may be properly controlled to reduce the coke yield and improve the coke selectivity. It is the zeolite in the composition of the FCC catalyst to have a deterministic impact on the comprehensive reaction performance of the catalyst. Therefore, it is a most effective means for improving the reaction performance of catalytic cracking catalyst to modify the zeolite.
Coke is composed of various hydrogen-deficient compounds having different hydrogen deficiency degrees. It is a product of the hydrogen transfer reaction. Therefore, in order to decrease the coke yield, it is required to reduce the hydrogen transfer reaction appropriately. The main course of hydrogen transfer reaction is the adsorption, reaction and desorption of protonated olefins on the acid sites of the zeolite. The higher the acid site density of the zeolite is, the intenser the hydrogen transfer reaction becomes. The acid site density of the zeolite is relevant to the Si/Al ratio in the skeleton of the zeolite. The lower the Si/Al ratio in the skeleton of the zeolite, the more the number of the acid sites of aluminum-oxygen tetrahedron, the higher the acid site density of the zeolite, the more the number of the hydrogen transfer reactions, the quicker the hydrogen transfer reaction, and the higher the coke yield; on the opposite, the higher the Si/Al ratio in the skeleton of the zeolite, the lower the acid site density of the zeolite, the less the number of the hydrogen transfer reactions, and the lower the coke yield. Therefore, in order to ensure that an active component has good coke selectivity, it is required for the active component to have a lower unit cell size and a suitable acid site density.
It is well known that in the operation of a cracking unit, in order to maintain a stable reaction activity, it is required to discharge the spent catalyst and supplement the fresh catalyst. Therefore, there exists a catalyst age distribution. The catalysts with different ages have different reaction performances. The zeolite in the fresh catalyst has larger unit cell size, therefore the fresh catalyst has higher cracking activity, stronger hydrogen transfer ability, and higher coke yield. For the catalyst in a long run, the zeolite is subjected to skeleton dealumination and structural collapse under the hydrothermal condition, and therefore the spent catalyst has lower cracking activity and poorer reaction selectivity. It is clear that the catalysts in these two conditions are both unfavorable for increasing the heavy oil utilization. In order to improve the heavy oil utilization of the catalyst, it should start from increasing the reaction performances of the zeolite under different deactivation degrees. In one hand, the zeolite having a low unit cell size can be used so as to decrease the coke selectivity of the fresh zeolite, and on the other hand, the activity-stability of the zeolite can be improved by modification so as to improve the equilibrium-activity and reduce the activity differences in the different stages caused by the hydrothermal aging of the zeolite, in order to reduce the coke selectivity of the catalyst wholly and therefore increase the heavy oil utilization.
Using the Y-type zeolite having a lower unit cell size consequently reduces the catalyst activity and the heavy oil conversion capability. Therefore, other modifying elements should be added to improve the performance of the active components. The modification with rare earth can remarkably increase the cracking activity and the hydrothermal stability of the zeolite. However, it is shown by many studies that the zeolite with high rare earth content has poor coke selectivity. Therefore, it is suitable to use the medium/low rare earth content. Recently, the modification by introducing both P and rare earth into the zeolite is adopted to improve the catalytic performance.
Patent literatures such as CN1157465C, CN1436727A, CN1506161A, CN1317547A, CN101537366A, EP0421422 and CN1951814A disclose the Si/Al ratio of the Y-type zeolite is increased by hydrothermal dealumination and/or chemical dealumination, and the shrinkage of unit cell is achieved by the second hydrothermal calcination. However, during the course of the deep dealumination (SiO2/Al2O3 molar ratio≧15), the zeolite structure is often destroyed to decrease the zeolite crystallinity.
U.S. Pat. No. 5,013,699 discloses a method of treating a Y-type zeolite, which method comprises NaY zeolite is subjected to ammonium ion exchange, and then high-temperature steam treatment. The resulting sample is then subjected to ammonium ion exchange at a pH of <4 and dealumination to obtain a zeolite product. According to that method, the zeolite sample is treated at a lower pH condition, and is not subjected to any protection. Therefore, the skeleton of the zeolite is prone to be destroyed, resulting in a decreased crystallinity of the zeolite.
U.S. Pat. No. 4,503,023 discloses a LZ-210 zeolite and its preparation method. Said method comprises a NaY zeolite is subjected to dealumination and silicon substitution with fluorosilicate to increase the Si/Al ratio of the zeolite. The resulting product has a higher crystallinity. However, when the Y-zeolite is dealuminated with fluorosilicate, the SiO2/Al2O3 molar ratio of the zeolite product is usually not higher than 13, otherwise the crystallinity of the zeolite product is remarkably decreased. In addition, the modified Y-zeolite prepared by dealumination and silicon substitution with fluorosilicate has few or no secondary holes, which is unfavorable for catalytically cracking the heavy oil.
CN1157465C discloses a catalytic cracking catalyst, which is composed of 50-95 wt % of the support and 5-50 wt % of the zeolite containing alkali-earth metal. That catalyst is prepared through mixing a compound containing alkali-earth metal with a zeolite homogenously in the presence of water optionally with the addition of aqueous ammonia, drying, calcining to obtain a zeolite containing alkali-earth metal, dispersing it in a support slurry, and drying-shaping.